CAF ⇝

constant applicative form

(CAF) A supercombinator which is
not a lambda abstraction. This includes truly constant
expressions such as 12, (+ 1 2), [1, 2, 3] as well as partially
applied functions such as (+ 4). Note that this last example
is equivalent under eta abstraction to \ x . + 4 x which is
not a CAF.

Since a CAF is a supercombinator, it contains no free
variables. Moreover, since it is not a lambda abstraction it
contains no variables at all. It may however contain
identifiers which refer to other CAFs, e.g.

c 3 where c = (* 2).

A CAF can always be lifted to the top level of the program.
It can either be compiled to a piece of graph which will be
shared by all uses or to some shared code which will overwrite
itself with some graph the first time it is evaluated. A CAF
such as

ints = from 1 where from n = n : from (n+1)

can grow without bound but may only be accessible from within
the code of one or more functions. In order for the garbage
collector to be able to reclaim such structures, we associate
with each function a list of the CAFs to which it refers.
When garbage collecting a reference to the function we collect
the CAFs on its list.